1. Field of the Invention
This invention relates to a process for working up ammoniacal solutions containing valuable metals, more especially copper-containing solutions, which reduces the consumption of ammonia and the unwanted discharge of salts in solvent extraction in relation to the prior art.
2. Discussion of Related Art
In the production of printed circuit boards for the electronics industry, the ammonium chloride solution used as the etching solution becomes enriched with copper during the continuous etching process. During this process, the copper content in such solutions increases continuously until, finally, a critical limit for the etching process is reached at a content of around 130 to 150 g of copper per liter. To enable processing of the circuit boards to be continued without interruption, excess copper has to be removed from the etching solution.
This can be done by various methods. For example, corresponding solutions are collected and concentrated by evaporation and, finally, the copper is recovered in the form of a salt, generally as copper hydroxide, by the addition of suitable precipitation reagents. Techniques such as these are expensive on energy and show poor selectivity and the copper obtained as end product is of inferior quality. Another disadvantage of this method is that there is no possibility of fully regenerating the spent etching solution and hence reusing it in the etching process.
EP-A-005415 discloses a process for removing copper from an etching solution used in solvent extraction. This process uses a weak extractant dissolved in an organic solvent which reduces the copper content in the etching solution after extraction from 160 g/l to around 97 g/l. .beta.-Diketones are preferably used as extractants in this process. The organic extraction solution is washed with water which gradually becomes enriched with large quantities of ammonium salts, ammonia and copper. Sodium hydroxide is added to the washing water which is then concentrated by evaporation, the ammonia being driven out and the copper being removed as copper hydroxide. The copper present in the organic phase is stripped with strong sulfuric acid and recovered from the aqueous solution by electrolysis. The process is operated on site, i.e. follows the etching process.
EP-A-036401 discloses a similar process. In this process, however, extraction is carried out in one or more steps with various extractants or mixtures thereof. For example, in addition to the .beta.-diketones known from EP-A-005415, hydroxy oximes are used as stronger extractants in this process. The copper content in the etching solution can thus be reduced to a total of 6.5 g/l. The process is extremely complicated on account of the need to use two separate extraction circuits and different reagents or mixtures thereof and/or to operate several different successive extraction stages and, for this reason, has never been adopted for practical application. In this process, too, the charged extraction solution is washed with mineral acid in order to free the organic phase from entrained ammonia.
Although the solvent extraction processes mentioned are more efficient by comparison with working up using precipitation reagents, they are still attended by certain serious disadvantages. For example, although the copper content in the extracted etching solution can be reduced to levels which, in principle, enable the etching process to be continued, the copper content is still very high at around 90 g/l or 6.5 g/l. This applies in particular to regeneration and reuse of the etching solution. The copper content cannot be reduced to values in the ppm range by these processes. Accordingly, the spent etching solution also cannot be completely worked up and reused which would be desirable in view of the total quantity of etching solution.
Although, with the introduction of new and more efficient extractants, such as for example LIX.RTM. 84, a ketoxime available from Henkel Corp., very low copper concentrations in the required ppm range can be reached in the purified etching solution without the use of extractant mixtures or separate, different extraction circuits, the complexing agents used have a tendency to complex free ammonia in a secondary reaction and to entrain it into the organic phase. However, in order to minimize the copper content of the extracted etching solution, the extractant has to be used in excess.
Now, a serious disadvantage of using such relatively strong extractants is that, in addition to the actual extraction of the copper into the organic phase, a troublesome unwanted effect is observed in the form of the extraction of free ammonia, which is always present in the spent etching solution, and the carryover of aqueous ammoniacal phase.
DE-A1-43 34 696 describes one such extraction process using a hydroxyoxime as a strong extractant. In this case, too, washing with acidified water is carried out to purify the organic phase, evaporation and subsequent recycling of the ammonium salt to the etching solution being mentioned as a working-up step for desalting the washing water after the extraction of ammonia. In addition to the considerable consumption of energy for evaporating the water, it is still necessary in the above-mentioned process to maintain a stream of solids which is more expensive than maintaining pure liquid streams.
This corresponds to the conventional practice of washing the organic extractant with aqueous dilute mineral acid, preferably HCl, until the ammonia has passed completely into the aqueous phase as ammonium salt for the purpose of removing ammoniacal impurities.
All the processes mentioned above are attended by the disadvantage that ammonium salts are constantly formed in aqueous solution and the concentration of ammonia in the etching solution decreases through the acidic washing of the organic solution to remove the ammonia. Either the ammonium salt has to be recovered from the aqueous phase so that it may be reused, which entails an additional separating step, or ammonia has to be continuously added to the etching solution which, as an additional cost factor, makes the process more uneconomical.
Although washing with hydrochloric acid leads to ammonium chloride, which may be returned to the etching process after further separation steps, this process is attended by the danger that chloride ions can be carried over into the stripping process and, hence, also into the electrolysis process for separating the valuable metal. This can give rise, for example, to serious disturbances in the metal extraction electrolysis process which are reflected in poorer quality of the valuable metal separated or which produce operational problems. Accordingly, efforts were made to carry out the first washing stage with sulfuric acid in order to minimize the danger of contamination by chloride ions. However, the disadvantage of this approach is that the ammonium sulfate formed cannot be fed back into the circuit and, hence, represents an additional cost factor both through the continuous consumption of ammonia and/or ammonium chloride and through the disposal costs for the ammonium sulfate.
Now, the problem addressed by the present invention was to provide a process for working up etching solutions containing valuable metals by which both the physically entrained ammonia and also the ammonia entrained in complexed form could be removed from the organic extraction solution and readily introduced into the circuit of the etching solution. Another problem addressed by the present invention was to minimize the discharge of salts in the form of ammonium salts. A further problem addressed by the invention was to select the process in such a way that it could be incorporated in already known metal extraction processes and would lead to substantially complete recycling of the etching solution as a whole.